A in the development of air transportation
B the earliest passenger flights were successfully operated
C to make travel easy and pleasant for the passengers
D to provide different services
E the shortage of qualified pilots
F traveling by air was very cheap
The forty years from the 1930s to the 1970s was an important periods______

A．will be the main transportation vehicles in the future.
B．will not be the main transportation vehicles in the future.
C．will be good to the environment in the future.
D．will replace petrol-powered vehicles in the futur

A．her
B．him
C．us
D．them